An integrated circuit consists of series of patterned functional layers (insulators, metal wires, etc). The structure of each layer is transferred from a mask via photolithography followed by etching or ion implantation. In the photolithographic process, the functional layer is covered by a photoresist film. The circuit patterns are fabricated with a chemically amplified photoresist consisting of a polymer with an acid-labile pendant protecting group, photoacid generator (PAG), and additional additives. Upon exposure to UV radiation through a patterned mask, the PAG is decomposed, generating a low concentration of acid. In the post-exposure bake, the acid diffuses and catalyzes a deprotection reaction that cleaves a pendant group of the insoluble polymer resulting in formation of a polymer that is soluble in the developer solution.
The exposed regions of the positive tone photoresist are removed by dissolution in a developer solution, generally using solutions including tetramethyl ammonium hydroxide in water, leaving a pattern of unexposed photoresist lines. To fulfill the demands for minimizing the feature size, the width of the photoresist structures must shrink adequately. Their heights cannot be reduced in the same way since etch resistance must be retained. With increasing aspect ratio, the mechanical strength of the photoresist lines decreases, leading to collapse of the structures during the development process. This pattern collapse is caused by unbalanced capillary forces acting between the lines after development and during the drying steps.
There have been a number of approaches developed to achieve reduction in feature size while preventing pattern collapse. The key parameters of the rinse fluid for preventing pattern collapse are the surface tension of the rinse fluid and the contact angle of the rinse fluid on the photoresist. (Tanaka, T.; Morigami, M.; Atoda, N.; (1993) Jpn. J. Appl. Phys.; Vol 32; pg 6059-6064). A promising approach to controlling surface tension and contact angle during the rinse step is to incorporate surfactant solutions in the development and/or rinse steps of the photolithographic process. The addition of a surfactant to the rinse liquid has been shown to reduce pattern collapse in several studies. Additional benefits include reducing defects and reducing line edge roughness (LER) and line width roughness (LWR). Surfactant rinses have been used to control the contact angle of aqueous fluids on the photoresist (U.S. Pat. No. 7,741,260) and reduce water marks (U.S. Patent Application Publication Nos. 2008/0280230 and 2008/0299487).
Several studies have discussed the importance of the adsorption of the surfactant on the photoresist surface (Tanaka, K., et al; (2003); Proc. of SPIE 5039, pp 1366-1381). Specifically, the use of a surface conditioning solution that interacts with the resist surface to smooth out the profile has also been discussed (Zhang, P. et al; (2006); Proc. of SPIE 6153; 61533Y). These studies demonstrate that the interaction of the surfactant with the photoresist, through adsorption of the surfactant onto or into the photoresist, is key for the reduction of LER, water defects and potentially for the other benefits of surfactant rinses.
However, high hydrocarbon surfactant concentrations may melt the photoresist and amplify pattern collapse (Watanbe, M.; Tomo, Y.; Yamabe, M.; Kiba, Y.; Tanaka, K.; Wakakizu, S; Kitano, J.; Yamada, Y.; (2004) Proc SPIE 5376:830). Additionally, the effect of a surfactant rinse is dependent on the photoresist (Pret, A. V; Gronheid, R.; Foubert, P.; (2010) J. Micro/Nanolith. MEMS and MOEMS; Vol 9; 041203). The addition of surfactants to the rinse liquid has many potential benefits, but identifying the optimal surfactant for a particular photoresist can be a complicated process. Other than surfactant concentration, the known candidates do not offer any process handle for tuning the surfactant interaction with the photoresist.
Furthermore, the surfactants in question must be sufficiently soluble in the rinse solution (e.g., water) to fully dissolve otherwise unwanted defects might be introduced onto the surface of the photoresist.